I recently put my Crowntail Betta in a newly planted tank.
The water parameters were absolutely fine and Felix settled in immediately. Quite frankly, he was enjoying himself.

Well today (not even a few days later) his tank was cloudy and some of the plants had browned (not all of them but a few unknown "loose plants" did). I assume since they were loose plants that they just didn't work well in the conditions I put them in, so I figured a quick cleaning would do the trick. Felix was hiding behind his filter, which was rather odd. I removed him from the tank and placed him in the temporary cup (the cup that I bought him from) and his breathing seemed really shallow--but I figured he was a bit stressed as his color was dulled out too which normally happens when I clean his tank. Well, I finished cleaning out the tank, scrubbing it thoroughly, and now Felix is on his side in the temporary cup and his breathing still seems very difficult for him.

I assume he isn't going to make it the rest of the evening. I figured I'd keep him in the cup as he has to literally "jump" up to get a breath of air and putting him back in his tank would only be harder for him to swim up.

Now here's the catch...I have the same exact tank, same filter, same conditions (water parameters, plants, lighting, etc) but with a different Betta (Ozwald) and his tank didn't get cloudy, Ozwald didn't get sickly, and his plants are the same, meaning the same couple loose plants aren't making it, but the rest are flourishing--but as said, Ozwald is absolutely fine.

Felix was eating the other day--and this morning I didn't have time to check on them as I had class first thing, but now I come home to a severely sickly fish and I'm unsure what's causing this? Is he just severely stressed or is it just his time? I figured I'd keep him in his cup to see if he looks up as I don't want to think about having to euthanize him if I absolutely don't have to.

Has anyone else had something like this happen? :3 Any recommendations on what to do?

Immediate water change. It's very possible something got into your tank that is toxic to both him and the plants. It's even possible that one of the plants had something nasty on it. I had a plant bulb in one of my tanks that I had to take out because it was making my tank smell like diesel oil and my fish was becoming lethargic.

How long have you had the sick betta? Was the tank set up and running prior to him being in it or did you put him in around the same time the tank was set up? Sometimes bettas are sick when you get them, which is why quarantining for 2 weeks is important. They sometimes will carry the illness and not show symptoms for some time. I would try filling out the form on this forum to help other people know about his tank and living conditions prior to him being sick. It also sounds like maybe something got into the water...check for any metal rods or anything that might be in the decor.

Immediate water change. It's very possible something got into your tank that is toxic to both him and the plants. It's even possible that one of the plants had something nasty on it. I had a plant bulb in one of my tanks that I had to take out because it was making my tank smell like diesel oil and my fish was becoming lethargic.

I don't know what would have killed him but not my other fish too? I tested the parameters a few times before and after he was in the tank and everything was fine. I mean, as said, the same exact types of plants were dying in my other tank as well (I assume it's about lighting), but it didn't result in my other fish becoming sickly.
*Note: I also don't just toss my fish in there right after I clean the tank. They're conditioned to it appropriately--just in case anyone thinks that may be the reason. XD

Potentially one of the plants had something wrong with it, but I tried eyeing them over really well before placing them in my aquarium. :3

I just don't get it. He was fine the other day >.> and it makes me feel as if I did something completely wrong. *sighs* thank you for the input though!

How long have you had the sick betta? Was the tank set up and running prior to him being in it or did you put him in around the same time the tank was set up? Sometimes bettas are sick when you get them, which is why quarantining for 2 weeks is important. They sometimes will carry the illness and not show symptoms for some time. I would try filling out the form on this forum to help other people know about his tank and living conditions prior to him being sick. It also sounds like maybe something got into the water...check for any metal rods or anything that might be in the decor.

I've had this Betta for a few months (since March 9th, I believe). And he doesn't have anything other than his live plants :3 nor have I ever had problems with his aquarium. Perhaps I just missed something--or did something wrong.

I just don't know what would have caused him to become sickly like he did.
Thank you for your input as well though!